Remove the Simple Default Stylesheet, it's just a foot-gun.

We've been bitten by the Simple Default Stylesheet being out
of sync with the real html.css twice this week.

The Simple Default Stylesheet was invented years ago for Mac:
http://trac.webkit.org/changeset/36135
It nicely handles the case where you just want to create
a single WebView and parse some simple HTML either without
styling said HTML, or only to display a small string, etc.

Note that this optimization/complexity *only* helps for the
very first document, since the default stylesheets are
all static (process-global) variables.  Since any real page
on the internet uses a tag not covered by the simple default
stylesheet, not real load benefits from this optimization.
Only uses of WebView which were just rendering small bits
of text might have benefited from this.  about:blank would
also have used this sheet.

This was a common application for some uses of WebView back
in those days.  These days, even with WebView on Android,
there are likely much larger overheads than parsing the
html.css stylesheet, so making it required seems like the
right tradeoff of code-simplicity for this case.

BUG=319556

Review URL: https://codereview.chromium.org/73723005

git-svn-id: svn://svn.chromium.org/blink/trunk@162153 bbb929c8-8fbe-4397-9dbb-9b2b20218538